1. Field of the Invention
This invention relates to an inexpensive, disposable oximetric sensor (e.g. a finger cot probe) that is non-adhesively attached to a human digit (e.g. a finger) to facilitate either the transillumination or transreflectance and the detection of electromagnetic (e.g. optical) energy through such digit to analyze the blood of a patient by calculating the concentration of blood constituents (e.g. the saturation of oxygen within the patient's blood) while minimizing potentially interfering noise artifact signals.
2. Background Art
To determine a characteristic within a space, electromagnetic energy is often transmitted through or reflected from a medium to determine its characteristics. In the medical field, instead of extracting material from a patient's body for testing, optical energy can be applied to human tissue so that transmitted or reflected energy can be measured to determine information about the material through which the energy has passed. This form of non-invasive measurement can be performed quickly and easily and has proven to be more comfortable to the patient.
Furthermore, non-invasive physiological monitoring of body functions is often required. For example, during surgery, the available supply of oxygen in the body, or the blood oxygen saturation, is often monitored. This measurement is sometimes performed by non-invasive techniques to enable medical determinations to be made by measuring the ratio of incident to transmitted (or reflected) light through a portion of the body such as, for example, a finger, an ear lobe, or the forehead. Transmission of optical energy as it passes through the body is strongly effected by the thickness of the material through which the energy passes, optical coupling, the optical angle, and the distance between the detector and the source of energy, collectively referred to as the optical path length.
Several parts of the human body are soft and compressible and ideally suited to transmit optical energy. For example, a human digit, such as the finger, comprises skin, muscle, tissue, bone, blood, etc. Although the bone is relatively incompressible, the tissue surrounding the bone is easily compressed when an external pressure is applied to the finger. However, if optical energy is applied to a finger and the patient moves or the finger is compressed in a manner which decouples the optical path between the optical source and detector, the optical path length correspondingly changes. Since a patient moves in an erratic fashion, the compression of the finger and the decoupling of the detector are also erratic. This causes the change in optical path length to be unpredictable and non-compensatable, making the absorption of optical energy erratic, thereby resulting in a noisy, difficult to interpret output signal.
Optical probes have been used in the past for both invasive and non-invasive applications. In the typical optical probe, a light emitting diode (LED) is placed on one side of the human tissue while a photodetector is placed on the opposite side. Such conventional optical probes are primarily useful when a patient is relatively motionless and in environments which are characterized by low ambient room light.
By way of particular example, one well known non-invasive measuring device in which an optical probe is used in health applications is the pulse oximeter which measures pulse rate and the percent of oxygen available in an arterial vessel. Up until the early 1980's, clinicians relied upon arterial blood gas analysis to evaluate gas exchange and oxygen transport within the human body. Although the arterial blood gas test gives valuable information, it only reflects a patient's oxygenation status for one moment in time. On the other hand, pulse oximetry permits a continuous, non-invasive measurement of a patient's arterial oxygen saturation status.
Oxygen saturation is defined as the amount of hemoglobin carried oxygen in relation to its total hemoglobin carrying capacity. Oxygen is carried by hemoglobin cells. A characteristic of hemoglobin is the different ways in which it absorbs both red and infrared light when carrying oxygen in the form of oxyhemoglobin relative to when it is not carrying oxygen in the form of reduced hemoglobin. Pulse oximetry takes advantage of this difference to determine arterial blood oxygen saturation.
An oximetric sensor commonly includes a photodetector and a pair of LEDs which emit both red and infrared light. The sensor is packaged in such a way that the LEDs and photodetector are placed on opposite sides of a vascular bed which, in the transillumination case, is usually a finger, ear lobe or toe. In the reflectance case, the LEDs and the photodetector are placed side by side, but separated by a barrier which blocks light from reaching the detector without first passing through the tissue sample. When properly positioned, the LEDs emit known wavelengths of both red and infrared light for transmission through the vascular bed for receipt by the detector.
As the photodetector receives unabsorbed light which passes through the vascular bed, a signal is produced. This signal is converted to digital form and then supplied to a computer or microprocessor which computes the ratio of red light to infrared light absorption. The absorption data is then utilized to determine the arterial blood oxygen saturation values which may then be displayed on a monitor or a strip chart. Since the light that is directed into the vascular bed is also at least partially absorbed by the nearby tissue and bone material, the oximeter utilizes the alternating bright and dim signals caused by arterial pulsations to further clarify the presence of both reduced hemoglobin and oxyhemoglobin.
By virtue of the foregoing, a health care provider is able to assess second to second changes in a patient's arterial oxygen saturation. This enables the possibility of intervention before hypoxemia occurs. Hypoxemia results from lack of oxygen in the blood which can lead to brain damage or even death. What is more, the health care provider is also able to evaluate the patient's response to treatment on a continuous basis.
Initially utilized in the operating room, pulse oximetry is becoming increasingly common in other parts of the hospital including emergency rooms, adult and neonatal intensive care units, and post anesthesia care units. It is expected that pulse oximeters will also find their way into the general ward and even outside the hospital by medical emergency technicians and private physicians. It is in these new areas that the prior art optical probes (i.e. sensors) have proven to be inadequate due to patient movement and their relatively noisy environment.
One conventional optical sensor that is adhesively attached to a patient's finger is disclosed in U.S. Pat. No. 4,830,014 issued May 16, 1989 to Goodman et al. In its non-applied configuration, this sensor has a plainer I-shape with adhesive covering an entire side. The area of the sensor which is intended to cover the radius of the finger is narrowed so as to provide less stability at the finger tip. This sensor is characterized by a very complex layered structure including a plurality of adhesive backed surfaces laid one atop the other. A first of said adhesive backed surfaces includes apertures through which a light source and optical detector communicate with one another. Another surface firmly engages the patient's finger so as to move therewith. In this sensor configuration, the light source must be precisely aligned with the apertures to insure that light will pass therethrough. As a consequence of the high degree of adhesive attachment between the sensor and the patient's finger, movement of the finger and the corresponding compression of the muscle tissue translates into tension, sudden optical decoupling, and compression of certain surfaces of the sensor.
The foregoing causes a shift in the light path length and a misalignment between the light source and detector. Further compression of the muscle tissue along one side of the finger with tension acting along the other side causes the light source to move relative to the detector along the entire length of the finger. This causes changes to the radiation angles and relocates the detector out of optimum alignment with respect to the light source.
Another known optical sensor is described in U.S. Pat. No. 5,125,403 issued Jun. 30, 1992 to Culp. A woven tube which is folded partially inside itself secures a side-folding light source and detector structure about a patient's finger tip. The finger engages the side-folding structure and pushes it inside the woven tube causing the tube to begin sliding inside out. However, the woven tube is unstable, tending to reverse its inside out movement. Moreover, the side-folding structure can slide off the tip of the finger thereby requiring that the entire assembly be refolded and refitted onto the finger. Flexing the finger can also cause disengagement, and the woven structure does not sufficiently act to straighten the finger after the finger has been flexed.